TWI843008B - Method and system for providing variable ramp-up control for an electric heater - Google Patents

Abstract

In one form, the present disclosure is directed toward a method for controlling temperature of a heater including a resistive heating element. The method includes applying power to the resistive heating element of the heater at a variable ramp rate to increase temperature of the heater to a desired temperature setpoint. The variable ramp rate is set to a desired ramp rate. The method further includes monitoring an electric current flowing through the resistive heating element of the heater, and reducing the variable ramp rate from the desired ramp rate to a permitted ramp rate in response to the electric current being greater than a lower limit of an electric current limit band. An upper limit of the electric current limit band is provided as a system current limit.

Description

Method and system for providing variable ramp control for electric heaters

This application claims priority to and the benefit of U.S. Provisional Application No. 63/064,523 filed on August 12, 2020. The disclosure of the above application is incorporated herein by reference.

Invention Field

This disclosure relates to controlling the temperature of a heater.

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

A thermal system generally includes a heater having a resistive heating element and a control system for controlling the power supplied to the heater to generate heat at a temperature set point. In one example application, a semiconductor processing system includes a thermal system having a pedestal heater including a heating plate having a ceramic substrate and one or more resistive heating elements defining one or more heating zones. The pedestal heater can be heated to different temperature set points to perform various processes, such as heating semiconductor wafers, a cleaning cycle, and other operations.

To achieve a temperature set point, the control system typically increases the temperature at a standard ramp rate (e.g., 5°C/minute, 10°C/minute, and others). The time spent changing the temperature set point typically idles the semiconductor chamber with the heater, which is lost or unproductive manufacturing time. These and other issues related to adjusting the temperature of a heater are addressed by the present disclosure.

This section provides a brief overview of the content of this disclosure and is not a comprehensive disclosure of its full scope or all of its features.

In one form, the present disclosure is directed to a method of controlling the temperature of a heater including a resistive heating element, and the method includes applying electrical power to the resistive heating element of the heater at a variable modulation rate to increase the temperature of the heater to a desired temperature set point, wherein the variable modulation rate is set to a desired modulation rate. The method further includes monitoring a current flowing through the resistive heating element of the heater, and in response to the current being greater than a lower limit of a current limit band, reducing the variable modulation rate from the desired modulation rate to an allowable modulation rate, wherein an upper limit of the current limit band is set to a system current limit.

In one variation, reducing the variable modulation rate further includes determining a reduction amount of the desired modulation rate based on a variable reduction factor, wherein the variable reduction factor increases as the current of the resistive heating element approaches the system current limit, and reducing the variable modulation rate to an allowable modulation rate by the reduction amount.

In another variation, the variable reduction factor provides a proportional reduction in the variable modulation rate based on the proximity of the current to the system current limit.

In yet another variation, the reduction is determined based on: RedAmt = (DesiredRate * %Reduction * RedFactor), where Reduction = 1.0-((ZoneCurLim-MeasuredCurrent)/CurrentBand), where: "RedAmt" is the reduction, "DesiredRate" is the desired modulation rate, "RedFactor" is the amount by which the variable modulation rate is reduced when the current is equal to the system current limit, "ZoneCurLim" is the maximum current limit of the resistive heating element, "MeasuredCurrent" is the measured current, and "CurrentBand" is the current limit band.

In one variation, the heater includes a plurality of resistive heating elements defining a plurality of zones, wherein each of the plurality of zones has a defined variable modulation rate.

In another variation, the current at each of a plurality of regions is monitored, and in response to at least one of the plurality of regions having a current greater than a lower limit of the current limit band, the variable modulation rate is reduced from a desired modulation rate to an allowable modulation rate.

In another variation, the method further includes determining a reduction amount for at least one region having a current greater than a lower limit of a current limit band based on a variable reduction factor, wherein the variable reduction factor increases as the current approaches a system current limit, and reducing a variable modulation rate for each of the plurality of regions based on the reduction amount to obtain an allowable modulation rate for each of the plurality of regions.

In one variation, the method further includes: monitoring a zone temperature of each of a plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; and adjusting a variable modulation rate of the first zone, the second zone, or a combination thereof in response to the difference being greater than the deviation threshold, wherein a zone of the first zone and the second zone having a higher zone temperature is set as a hot zone, and the other zone of the first zone and the second zone is a cold zone.

In another variation, to adjust the variable modulation rate, the variable modulation rate of the hot region is reduced, the variable modulation rate of the cold region is increased, or a combination thereof.

In yet another variation, to adjust the variable modulation rate, the method further includes reducing the variable modulation rate of the hot zone to zero, maintaining the zone temperature of the hot zone until the difference is no longer greater than a deviation threshold, and increasing the variable modulation rate of the hot zone in response to the difference being less than the deviation threshold.

In one variation, the method further includes: setting the variable modulation rate to a sliding control rate, wherein the sliding control rate is a modulation rate less than the desired modulation rate; and increasing the variable modulation rate to the desired modulation rate in response to a sliding condition being satisfied, wherein the sliding condition includes a predetermined time, the temperature of the heater being equal to a sliding temperature set point less than the desired temperature set point, or a combination thereof.

In another variation, the method further includes determining whether the temperature of the heater is at a temperature approaching a critical value, wherein the temperature approaching the critical value is less than the desired temperature set point, and in response to the temperature of the heater being at the temperature approaching the critical value, reducing the variable modulation rate to an approaching modulation rate, wherein the approaching modulation rate is less than the desired modulation rate.

In one form, the present disclosure is directed to a control system for controlling power supplied to a heater including a resistive heating element, and the control system includes a processor and a non-transitory computer-readable medium, the non-transitory computer-readable medium including instructions executable by the processor, wherein the instructions include determining the amount of power to be supplied to the resistive heating element of the heater based on a variable modulation rate that increases the temperature of the heater to a desired temperature set point, wherein the variable modulation rate is set to a desired modulation rate. The instructions further include monitoring a current flowing through the resistive heating element of the heater and, in response to the current being greater than a lower limit of a current limit band, reducing the variable modulation rate from the desired modulation rate to an allowable modulation rate, wherein an upper limit of the current limit band is set to a system current limit.

In one variation, the instructions further include determining a reduction amount of the desired modulation rate based on a variable reduction factor, wherein the variable reduction factor increases as the current of the resistive heating element approaches the system current limit; and reducing the variable modulation rate by the reduction amount to obtain an allowable modulation rate.

In another variation, the variable reduction factor provides a proportional reduction in the variable modulation rate based on the proximity of the current to the system current limit.

In yet another variation, the heater includes a plurality of resistive heating elements defining a plurality of regions, wherein each of the plurality of regions has a defined variable modulation rate.

In one variation, the current at each of a plurality of regions is monitored, and in response to at least one of the plurality of regions having a current greater than a lower limit of the current limit band, the variable modulation rate is reduced from a desired modulation rate to an allowable modulation rate.

In another variation, the instructions further include determining a reduction amount for at least one region having a current greater than a lower limit of the current limit band based on a variable reduction factor, wherein the variable reduction factor increases as the current approaches the system current limit, and reducing the variable modulation rate of each of the plurality of regions based on the reduction amount to obtain an allowable modulation rate for each of the plurality of regions.

In yet another variation, the instructions further include: monitoring a zone temperature of each of a plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; and in response to the difference being greater than the deviation threshold, adjusting the variable modulation rate of the first zone, the second zone, or a combination thereof, wherein a zone of the first zone and the second zone having a higher zone temperature is set as a hot zone, and the other zone of the first zone and the second zone is a cold zone.

In one variation, to adjust the variable modulation rate, the instructions further include decreasing the variable modulation rate of the hot region, increasing the variable modulation rate of the cold region, or a combination thereof.

In another variation, the instructions further include reducing the variable modulation rate of the thermal zone to zero to maintain the zone temperature of the thermal zone until the difference is no longer greater than the deviation threshold; and in response to the difference being less than the deviation threshold, increasing the variable modulation rate.

In yet another variation, the instructions further include: setting the variable modulation rate to a sliding control rate, wherein the sliding control rate is a modulation rate less than the desired modulation rate; and increasing the variable modulation rate to the desired modulation rate in response to a sliding condition being satisfied, wherein the sliding condition includes a predetermined time elapsed, the temperature of the heater being equal to a sliding temperature set point less than the desired temperature set point, or a combination thereof.

In one variation, the instructions further include determining whether the temperature of the heater is at a temperature approaching a critical value, wherein the temperature approaching the critical value is less than the desired temperature set point; and in response to the temperature of the heater being at the temperature approaching the critical value, reducing the variable modulation rate to an approaching modulation rate, wherein the approaching modulation rate is less than the desired modulation rate.

Further scope of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for illustrative purposes only and are not intended to limit the scope of the present disclosure.

100: Thermal system

102: Pedestal heater, heater

104: Control system

106: Controller

108: Power converter system

110: Heating plate

111: Matrix

112: Support shaft

114: Heating area, area, given area

115: Terminal

117: Computing device

118: Power supply

120: Interlock device

124: Sensor circuit

126:Ammeter

128:Voltmeter

130: Variable rate temperature control, VRRT control

200: VRRT control routine, routine

202,204,206,208,210,212,302,304,306,308,310,312,314,402,404,406,408,410: Steps

300: Variable pitch control, routine

400: Variable pitch drop control, routine

In order to make the present disclosure well understood, its different forms will now be described by way of example and with reference to the accompanying drawings, wherein: FIG. 1 illustrates a thermal system according to the present disclosure, which has a heater and a control system with a variable modulation rate temperature control; FIG. 2 is a flow chart of an exemplary variable modulation rate temperature control according to the present disclosure; FIG. 3 is a flow chart of an exemplary variable modulation rate control of FIG. 2; FIG. 4 is a flow chart of an exemplary variable modulation rate control of FIG. 2; FIG. 5A is A diagram of a constant up control according to the present disclosure; FIG. 5B is a diagram of a variable up control according to the present disclosure; FIG. 6 is a diagram of a variable up control according to the present disclosure; and FIG. 7A and 7B are diagrams of variable down control according to the present disclosure; FIG. 8 is a diagram of a variable down control for suppressing regional drift conditions according to the present disclosure; and FIG. 9 is a diagram of a variable down control for mitigating out-of-control conditions according to the present disclosure.

The drawings described herein are for illustrative purposes only and are not intended to limit the scope of this disclosure in any way.

The following description is merely exemplary in nature and is not intended to limit the content, application or use of the present disclosure. It should be understood that in all figures, corresponding reference numbers indicate similar or corresponding parts and features.

Referring to FIG. 1 , a thermal system 100 includes a heater 102 and a control system 104 having a controller 106 and an electric power converter system 108. In one form, the heater 102 includes a heating plate 110 and a support shaft 112 disposed at a bottom surface of the heating plate 110. The heating plate 110 includes a substrate 111, and a plurality (i.e., "plurality" means two or more) of resistive heating elements (not shown) embedded in or disposed along a surface of the substrate 111. In one form, the substrate 111 may be made of ceramic or aluminum. The resistive heating elements are independently controlled by the control system 104 and define a plurality of heating zones 114, as illustrated by the dotted lines in FIG. 1 . It should be readily appreciated that the heating zones 114 may be configured differently and still be within the scope of the present disclosure. Additionally, the pedestal heater 102 may include one or more zones and should not be limited to a multi-zone heater.

In one form, the heater 102 is a "two-wire" heater in which the resistive heating element functions as both a heater and a temperature sensor, and only two lead wires are operably connected to the resistive heating element instead of four. Such two-wire capabilities are disclosed, for example, in U.S. Patent No. 7,196,295, which is commonly assigned with the present application and is incorporated herein by reference in its entirety. Generally, in a two-wire system, the resistive heating elements are defined by a material that exhibits a variable resistance with varying temperature, such that an average temperature of the resistive heating element is determined based on a change in the resistance of the resistive heating element. In one form, the resistance of the resistive heating element is calculated by first measuring the voltage across the heating element and the current through the heating element, and then using Ohm's law to determine the resistance. Using a resistance-to-temperature conversion data (e.g., a table, an algorithm, etc.), a temperature of the resistive heating element and thereby the region 114 is determined (i.e., a region temperature). The resistive heating element may be defined by a relatively high temperature coefficient of resistance (TCR) material, a negative TCR material, or in other words, a material having a nonlinear TCR.

The control system 104 controls the operation of the heater 102, and more particularly, is configured to independently control the power provided to each of the zones 114. In one form, the control system 104 is electrically coupled to the zones 114 via terminals 115 such that each zone 114 is coupled to two terminals that provide power and sense temperature.

In one form, the control system 104 is communicatively (e.g., wirelessly and/or wiredly) coupled to a computing device 117 having one or more user interfaces, such as a display, a keyboard, a mouse, a speaker, a touch screen, and others. Using the computing device 117, a user can provide input or commands, such as temperature set points, power set points, and/or commands to execute a test or a program stored by the control system 104.

The control system 104 is electrically coupled to a power source 118, which supplies an input voltage (e.g., 240V, 208V) to the power converter system 108 through an interlock device 120. The interlock device 120 controls the power flowing between the power source 118 and the power converter system 108, and can be operated by the controller 106 as a safety mechanism to cut off the power from the power source 118. Although illustrated in FIG. 1, the control system 104 may not include the interlock device 120.

The power converter system 108 is operable to regulate the input voltage and apply an output voltage (V _OUT ) to the heater 102. In one form, the power converter system 108 includes a plurality of power converters (not shown) operable to apply an adjustable power to a resistive heating element of a region 114. An example of such a power converter system is described in U.S. Patent No. 10,690,705, entitled “Power Converter for Thermal Systems,” which is commonly owned by the present application and is incorporated herein by reference in its entirety. In this example, each power converter includes a buck converter that can be operated by the controller 106 to produce a desired output voltage that is less than or equal to the input voltage for one or more heating elements in a given zone 114. Accordingly, the power converter system 108 can be operated to provide a customizable amount of power (i.e., a desired power) to each zone 114 of the heater 102. Other power converter systems configured to provide adjustable power to the heater 102 can also be used and should not be limited to the examples provided herein. For example, the power converter system can be an isolated power converter system for providing an isolated power output to the heater. An example of such a power converter system is described in U.S. Patent No. 11,038,431, entitled "Isolated Power Converter for Thermal Systems," which is commonly owned with the present application and is incorporated herein by reference in its entirety.

By using a two-wire heater, the control system 104 includes sensor circuits 124 to measure electrical characteristics (i.e., voltage and/or current) of the resistive heating element, which are then used to determine the performance characteristics of the regions 114, such as resistance, temperature, current, voltage, power, and other suitable information. In one form, a given sensor circuit 124 includes an ammeter 126 and a voltmeter 128 for respectively measuring a current flowing through and a voltage applied to the heating element in a given region 114. In another form, the voltage and/or current measurements may be made at zero crossings, as described in U.S. Patent No. 7,196,295.

Instead of or in addition to a "two-wire heater", the thermal system 100 may include discrete sensors for measuring characteristics of the heater 102 (e.g., voltage, current, and/or temperature) and providing respective data to the controller 106. For example, in one form, at least one voltmeter and ammeter may be configured to measure electrical characteristics of the zone 114 (e.g., voltage and current), and at least one temperature sensor may be configured to measure a temperature of the heater and/or the temperature of each zone 114.

In one form, the controller 106 includes one or more microprocessors and a memory for storing computer readable instructions executed by the microprocessor. In one form, the controller 106 is configured to implement one or more control programs, wherein the controller 106 determines the desired power to be applied to the zone 114, such as 100% of the input voltage, 90% of the input voltage, etc. Example control programs are described in U.S. Patent No. 10,690,705 (referenced above), and U.S. Patent No. 10,908,195 entitled "System and Method for Controlling Power Sent to a Heater", which are commonly owned with the present application and are incorporated herein by reference in their entirety. In one form, the controller 106 implements a closed loop temperature control in which the temperature of the heater is controlled to a temperature set point. For example, using the resistance of the resistive heating element and a calibrated resistance-temperature model, the controller 106 determines a temperature of the zones 114 and then adjusts the power supplied to the zones 114 to bring the temperature of the zones 114 closer to the temperature set point.

In one form, the control process also includes a variable modulation rate temperature (VRRT) control 130, wherein the heater 102 initially undergoes a variable temperature modulation rate to reach a temperature set point. Once at the temperature set point, the controller provides a steady-state closed loop control to maintain the temperature of the heater 102 at the temperature set point. In certain applications, the heater 102 may be controlled to different temperature set points for an industrial process, and at times, the temperature may fluctuate and run from a first temperature to a second temperature that is lower than the first temperature.

In one form, the VRRT control 130 is configured to provide a variable ramp-up control to increase the temperature of the heater 102, and a variable ramp-down control to decrease the temperature of the heater 102. Although the VRRT control 130 is configured to have both, the VRRT control 130 may include one of the variable ramp-up control and the variable ramp-down control, and need not have both.

The variable ramp-up control is configured to provide power to the resistive heating element of the heater 102 at a variable ramp-up rate to increase the temperature of the heater 102 to a temperature set point. The variable ramp-up rate is defined based on the current provided to the heater 102 and, in the case of a multi-zone heater, the temperature of the zone 114. More specifically, to inhibit damage to components of the thermal system 100, such as a power switch, power converter, wiring, and/or fuses, among others, the current applied to the heater 102 is controlled to be below a system current limit, which may be a zone current limit and/or a heater current limit. For example, for the zones 114, the current delivered to each zone 114 is monitored and controlled for each zone 114 to be below a zone current limit that is a system current limit. In one form, for a multi-zone heater, the current at one zone can affect the current at other zones. That is, to provide coherent modulation, as a single zone approaches the system current limit, the variable ramp control adjusts (e.g., decreases) the variable ramp rate of all zones at the same decreasing rate. For a temperature set point, the variable ramp control defines a system current limit (i.e., the maximum allowable current of the heater and/or the zones); and a desired modulation rate, which is a maximum desired modulation rate of the variable ramp rate.

To provide a co-regulated temperature profile for a multi-zone heater, the variable ramp control monitors and controls the temperature of the zones 114 so that the temperature difference between any two zones 114 (e.g., a first zone and a second zone) is less than a deviation threshold (a zone-to-zone drift/deviation). More specifically, the regulation is controlled by a moving set point (i.e., a temperature regulation set point (TempRampSP)) that moves at a rate set point (RateSP, i.e., a variable modulation rate). That is, in one form, the rate set point is °C per minute and is the rate at which the TempRampSP changes. The TempRampSP is an absolute temperature that the controller 106 maintains a measured temperature to reach as it moves using, for example, proportional-integral-derivative (PID) control. The measured temperature may be referred to as a process value (PV). Since the TempRampSP is continually moving until it reaches the temperature set point, the process value should also move. In one form, the integral time constant in the PID is responsive to establish power to match the rate set point. In one form, if the process variable of any one zone deviates from the process variable of the other zones 114, the variable ramp control adjusts the RateSP of one or more zones to provide a synchronized temperature control of the heater 102. In one form, the variable ramp control may reduce the RateSP of the zone that deviates from the other zones to provide a synchronized temperature profile. In another form, the variable ramp control may increase the RateSP of other zones to improve the performance of those zones 114 while monitoring the current of the zones 114.

For variable ramp control, Table 1 provides the control variables used to control the ramp rate based on current and temperature:

In one form, to control the modulation rate based on current, a variable modulation rate control sets the variable modulation rate for a zone based on a measured current in the zone and the total current to the heater. In particular, the variable modulation rate is set just high enough to stay below the system current limit (e.g., the zone current limit and/or the heater current limit). The variable modulation rate is initially set to a desired modulation rate, and if the zone current limit is within the current limit band, the variable modulation rate is reduced from the desired modulation rate to an allowable modulation rate based on a calculated reduction amount. Except for the zone near the zone current limit, the variable modulation rate of the other zones is reduced by the same reduction amount to provide coherent current control. The amount of reduction depends on how close the measured current is to the system current limit, so that the smaller the difference between the measured current and the system current limit, the higher the reduction.

More specifically, the variable ramp-up control defines a proportional reduction amount for a current limit band based on a percentage of a reduction factor and the difference between a measured current and a system current limit such as a regional current limit. That is, in one example application, the proportional reduction is based on the proximity of the current to the system current limit. For example, the reduction amount is determined using equations 1 and 2, where "% reduction" is set to a variable reduction factor that increases as the measured current of a resistive heating element in a region approaches the regional current limit.

Equation 1.....RedAmt=(DesiredRate * %Reduction * RedFactor)

Equation 2.....% reduction = 1.0-((ZoneCurLim-MeasuredCurrent)/CurrentBand)

As provided in Equation 2, the variable reduction factor is configured to provide a proportional reduction such that if the measured current is below the current limit band, the reduction factor is 0%; if the measured current is within the current band, it is between 0-100%; if the measured current is equal to the system current limit, it is 100%; and if the measured current is greater than the system current limit, it is greater than 100% to provide a greater reduction than the reduction factor. In one form, if the measured current is above the regional current limit, the variable modulation rate continues to be reduced to a nominal rate such as 1°C/minute or other suitable value to prevent shutdown.

In one form, to control the rate of change based on temperature, the variable ramp control measures the temperature of each zone and initially sets the temperature change set point for a zone to a respective measured temperature value to suppress temperature jumps. From this point, the temperature will begin to increase toward the temperature set point. The temperatures of the zones are measured regularly, and if the temperature of a zone begins to deviate from the other zones (i.e., too high or too low), the variable temperature ramp rate is adjusted to provide a uniform temperature. In one form, the variable ramp control reduces the rate of change for the zone closest to the temperature set point (i.e., the hot zone) to allow the other zones (i.e., the cold zone) to catch up to the temperature change set point of the hot zone. The amount of reduction is selected to provide a responsive reduction, but not so radical as to reduce heating operations. For example, for each degree of deviation, the modulation rate may be reduced by 5-15%. In another form, while monitoring the current supplied to the heater and the zones, the variable ramp-up control increases the modulation rate of the cold zone to allow the cold zone to catch up to the temperature adjustment set point of the hot zone. For example, the modulation rate of the cold zone may be increased in set increments (e.g., 1°C/minute, 2°C/minute, 0.5°C/minute increments). In this enhancement method, the variable ramp-up control may also reduce the modulation rate of the hot zone, or maintain the temperature of the hot zone at the current temperature adjustment set point until the other zones are at or near the measured temperature of the hot zone.

In one form, at the start of control, the variable ramp-up control may provide a sliding control for controlling the speed of the modulation rate change; and an approach control to reduce or suppress a surge in temperature when approaching a temperature set point. More specifically, the modulation rate is set to a sliding control rate that is a modulation rate significantly lower than the desired modulation rate (e.g., sliding control rate = 1.0°C/minute). In one form, the modulation rate is maintained at the sliding control rate until a sliding condition is met, wherein the sliding condition may include, for example, a predetermined time and/or a desired temperature modulation set point (i.e., a sliding temperature set point) is reached. Thereafter, the variable modulation rate is increased to the desired modulation rate. In one form, the sliding control rate is always applied when the modulation rate changes to control the acceleration of the modulation rate.

The approach control is configured to reduce the modulation rate to an approach modulation rate when the measured temperature is a defined distance/range from the final temperature set point (i.e., a temperature approaches a critical value). The modulation rate is reduced to allow the heater to reach the temperature set point without overshooting the temperature set point. In one form, the approach control is applied when approaching the temperature set point (e.g., during ramp-up or ramp-down) to allow the integration time to approach a value appropriate for the temperature set point. For example, if the factor is 1.0, the reduction is from the temperature set point by the modulation rate degree. Accordingly, a reduction of 10°C/minute is a 10°C reduction from the temperature set point.

Variable droop control is configured to provide a coordinated cooling of the heater to a temperature set point less than the measured temperature. For semiconductor processes, the rate at which the heater cools may be a function of the chamber, and the rate may decrease as the temperature decreases and/or as the walls of the chamber are heated. For a multi-zone heater, different zones of the heater may cool at different rates when power is removed or significantly reduced. To reduce the temperature differences between the zones, variable droop control is configured to cause the variable rate to be at or above the natural droop rate (i.e., the rate of decrease without power).

In one form, the variable ramp down control reduces the temperature of the zones with a cooling variable ramp rate such that the temperature set point is continuously reduced at a defined rate. For example, in one form, the cooling variable ramp rate is first set to a desired cooling ramp rate, such as 10°C/minute, and the temperatures of the zones are monitored to maintain a uniform thermal profile of the heater during the cooling period.

To provide a coherent thermal profile, the variable derating control determines if one or more of the following runaway conditions exist: a region-to-region drift, a setpoint deviation, and/or a region float condition. If a runaway condition is detected, the variable derating control applies a corrective action.

For zone-to-zone drift, variable derating control determines if one zone is cooling faster or slower than other zones. Specifically, in one form, the variable derating control determines if a temperature of a target zone is within a zone deviation threshold of other zones. To reduce deviation and provide consistent derating, if the target zone is deviating from one or more other zones, the variable derating rates of all zones are adjusted as a corrective action.

In response to a modification set point deviation, the variable derating control determines whether a zone has fallen too far behind a temperature modification set point during derating. Specifically, during derating, the temperature modification set point is continuously decreased according to a variable modulation rate. If the temperature of the target zone is lagging (i.e., not cooling fast enough), the modulation rate is adjusted so that the temperature of the target zone continues to decrease while allowing the target zone to catch up with the temperature modification set point. In form, to detect a modification set point deviation, the variable derating control determines whether the temperature of the target zone has deviated from the temperature modification set point by a value greater than or equal to a set point deviation threshold value (i.e., a deviation threshold value). If so, a set point deviation condition has been detected.

To mitigate a zone-to-zone drift and/or a modified set point deviation, the variable derating control reduces the variable modulation rate to a value less than the desired modulation rate value (e.g., from 10°C/min to 5°C/min) as a corrective action. In one form, the variable derating control determines the reduction amount (i.e., a modified cooling reduction amount (RCoolRedAmt)) based on the deviation between the temperature of the zone and/or the temperature modified set points of other zones. For example, in one form, the reduction is determined using equations 3-5, where: PVH is the measured temperature of the hot zone; PVL is the measured temperature of the cold zone; WeightPara1 is a weighting parameter for the delta measured temperature and is provided as a reduction per degree of deviation (e.g., 10%/°C); and WeightPara2 is a weighting parameter for the difference between the cold zone and the temperature adjustment set point and is provided as a reduction per degree of deviation (e.g., 5%/°C). Once determined, the adjustment cooling reduction is applied to each of the zones.

Equation 3.....RCoolRedAmt=Regional Deviation Reduction+Setpoint Deviation Reduction

Equation 4....Regional deviation reduction = |(PVH-PVL)|*WeightPara1

Equation 5.....Set point deviation reduction = |(PVL-TempRampSP)|*WeightPara2

In one variation, the amount of the modified cooling reduction is based on one of a region deviation reduction or a set point deviation reduction (i.e., a set point deviation amount). For example, if there is only one region-to-region drift, then a reduction in the set point deviation may not be necessary. Alternatively, if both of the deviation conditions exist, the variable derating control may first reduce the deviation of the region-to-region drift based on the region deviation reduction, and until the deviation between the regions is within a critical value. Thereafter, the amount of the modified cooling reduction is determined using both the region deviation reduction and the set point deviation reduction as provided in Equation 3. It should be readily understood that the values provided herein are for explanation purposes only and may be any suitable values.

In another form, if the temperature of at least one zone begins to deviate from the other zones, the temperature adjustment set point of the cold zone is set to the measured temperature of the hot zone. That is, the variable derating control increases the power given to the zone with the lower temperature to increase the temperature of the zone to the temperature of the zone with the higher temperature. Accordingly, the variable derating control brings the temperatures of the zones together or within a deviation threshold (e.g., ±5°C), which can flatten the temperature adjustment set point curve before the zone approaches the temperature set point.

In a zone floating condition, variable droop control determines whether a zone is floating or swimming. More specifically, as power to a zone decreases, it may be difficult to accurately measure a process value (e.g., temperature), and in some cases, the power may be so low that the zone may be uncontrollable (e.g., the power is at a minimum power level/output that is greater than zero volts but insufficient to control the zone). That is, the temperature of the zone may begin to deviate from the temperature adjustment set point, and if there are multiple zones, the temperature of the zone may begin to deviate from another zone. To control derating during the zone floating condition, a variable derating control is configured to increase power to the zone experiencing the floating condition to a nominal power output (e.g., 2% power, 5% power) that is greater than the minimum power level (i.e., minimum power output) to obtain control of the zone while still reducing the temperature of the zone. In one form, power is increased by decreasing the variable modulation set point until power is again applied at the nominal power output. The nominal power output applied to the zone to suppress the zone floating condition may be defined based on testing and may be just above the minimum power level (e.g., nominal power output is above 5V).

If more than one of the out of control conditions is detected, the reduction is a weighted combination of the reductions for the detected deviation conditions. In one form, the weight assigned to each deviation condition may be based on which stage in the cool down sequence the heater is in. That is, typically, a regulation set point deviation occurs earlier in a cool down of the heater than a zone to zone drift, which may occur as the heater gets cooler. Accordingly, when the heater is first beginning to cool down, the reduction associated with the regulation set point deviation is assigned a higher weight than the reduction associated with the zone to zone drift. After a certain time and/or after the temperature of the heater reaches a selected temperature set point greater than the desired temperature set point, the variable derating control may assign a higher weight to the reduction associated with the zone-to-zone drift than to the modified set point deviation. At colder temperatures, power to the heater may no longer be required, so a minimum amount of power may be applied to suppress zone floating conditions, which may take precedence over the zone-to-zone drift and the modified set point deviation. Accordingly, weighting factors may be assigned based on the stage of the heater in cooling and the heater itself (i.e., the responsiveness of the heater).

It should be readily appreciated that variable droop control can be configured to monitor one or more out-of-control conditions and need not monitor all. For example, for a single zone heater, zone-to-zone drift is not required.

2, an example VRRT control routine 200 is provided and implemented by a control system to control the temperature of a heater to one or more temperature set points. In step 202, the control system obtains a temperature set point for the heater from, for example, a defined state mode that provides the temperature set point and duration of the heater. In step 204, a determination is made as to whether the temperature set point is less than the current temperature of the heater. If the temperature set point is higher, then in step 206, the control system implements variable ramp-up control. On the other hand, if the temperature is lower, then in step 208, the control system implements variable ramp-down control. Once the temperature set point is reached, at step 210, the control system returns to routine 200, uses a temperature control model (e.g., a PID control) to maintain the temperature at the temperature set point, and at step 212, determines whether there is a new temperature set point. If there is a new temperature set point, the control system returns to step 202. In one form, the temperature set point may include a nominal set point when the heater is then turned off.

Referring to FIG. 3 , an example variable ramp control 300 is provided. In step 302, the control system sets the variable ramp rate to a desired ramp rate defined based on a temperature set point and/or system current limit, and provides power to the heater to achieve the desired ramp rate. In step 304, the control system monitors the current flowing through the resistive heating element through multiple zones and the temperature of each zone. In step 306, the control system determines whether the measured current of each zone is less than the current limit band. If so, the control system proceeds to step 310. If not, in step 308, the control system determines a reduction factor and reduces the variable ramp rate of each zone based on the reduction factor. In particular, using the design method described above, the control system determines a reduction factor that is correlated to the proximity of the measured current to the system current limit and reduces the variable modulation rate by a reduction amount to obtain an allowable modulation rate, which is used as the variable modulation rate for each zone. In step 310, the control system determines whether the temperature of the adjacent zones is within a deviation threshold to maintain a coherent temperature profile of the heater. If the temperatures are within the deviation threshold, the control system continues to step 314. If at least one zone is in deviation, then in step 312, the control system reduces its variable modulation rate for the zone with the higher temperature, as provided above. Alternatively, the control system may be configured to increase power to other zones while monitoring the current in those zones. In step 314, the control system determines whether the zones are at the temperature set point. If not, the control system returns to step 304. If the zones are at the temperature set point, the control system returns to routine 200 of FIG. 2.

4 , an example variable modulation reduction control 400 is provided. At step 402, the control system sets the variable modulation rate to a cooling modulation rate (e.g., a second variable modulation rate) and controls the zones based on the cooling modulation rate. At step 404, the control system monitors the temperature of each zone, and at step 406, the control system determines whether a temperature difference between the zones is within a deviation threshold to provide a coherent temperature profile when the heater cools to the temperature set point. For example, the control system determines whether the temperature difference between adjacent zones is greater than the deviation threshold. If the temperature differences are within the deviation threshold, the control system proceeds to step 410. If at least one zone is in deviation, then in step 408, the control system sets the temperature adjustment set point of the zone with the lower temperature (i.e., the cold zone) to the measured temperature of the zone with the higher temperature (the hot zone), and therefore, increases the power supplied to the cold zone to achieve the new temperature adjustment set point. In step 410, the control system determines whether the zones are at the temperature set point. If not, the control system returns to step 404. If the heater is at the temperature set point, the control system returns to routine 200 of FIG. 2.

It should be readily appreciated that routines 200, 300, and 400 can be configured in a variety of suitable ways and should not be limited to the steps described herein. For example, if the heater is a single zone heater, the control system may skip the steps associated with providing a coherent temperature profile in routine 300, and the variable derating routine may be omitted. In another example, the VRRT control also includes a sliding speed control and/or a proximity control to provide a smooth transition to a desired modulation rate and to a temperature set point, respectively. In yet another example, for the variable derating control, instead of setting a cooling modulation rate, the control shuts off power to the heater and monitors the temperatures of the zones to mitigate possible temperature deviations.

Figures 5A to 9 illustrate the nature of VRRT control of the present disclosure. Specifically, Figure 5A illustrates a ramp-up operation in which the ramp rate is constant (e.g., 20°C/min), and Figure 5B illustrates a ramp-up operation using VRRT control of the present disclosure. In both, the current is maintained below 30A, but the constant ramp rate of Figure 5A takes longer to reach 600°C than the VRRT control of Figure 5B. For the VRRT control, the ramp rate starts at 28°C/min and decreases as the current approaches 30A. That is, once the measured current is within a current limit band (e.g., 25-30A), the ramp rate is reduced to control the current applied to the heater while allowing the heater to reach the temperature set point.

FIG6 is a diagram illustrating ramp control, where the ramp rate is controlled from a sliding speed (GlideSpeed) to a desired ramp rate, and then to an approach control rate (ApproachControl) when the measured temperature approaches the temperature set point. RampSP refers to the ramp set point, RampingSP1 refers to the first ramp set point, and ActualRateSP1 refers to the first actual rate set point.

Figures 7A and 7B are graphs illustrating cooling of a two-zone heater without variable droop control and with variable droop control, respectively. As illustrated in Figure 7A, the zone temperatures begin to deviate from each other, which can cause thermal stress, while in Figure 7B, the heater has a co-regulating temperature profile by handling the deviating temperatures. FilamentTemp1 and FilamentTemp2 refer to the first and second filament temperatures of the first resistive heating element and the second resistive heating element at the first and second zones of the two-zone heater, respectively. RequestedHtrVolts1 and RequestedHtrVolts2 refer to the requested first heater voltage and the requested second heater voltage, respectively.

FIG8 illustrates a variable ramp down control of VRRT control, where the heater with two zones is powered at a level slightly above the minimum amount (e.g., 5% power is provided) to suppress zone floating conditions. By providing a small amount of power to the heater, the temperature of the heater is continuously monitored and still reduced to the temperature set point. RampingSP1 refers to the first ramp set point, Wattage1 refers to the power delivered to the first zone, Wattage2 refers to the power delivered to the second zone, and PidEffort1 is the total output of the PID control loop, which has a value between 0 and 100% of the available power (provided in "%).

FIG9 illustrates variable ramp down control, where runaway conditions are reduced or suppressed by controlling ramp rate and/or power, as described above. In the figure, the filament temperature (FilamentTemp1) representing the heater temperature and the temperature ramp set point (i.e., the ramp set point (RampingSP1) in FIG9) are substantially the same during ramp down. In the figure, the optimal adjustment process variation reduction (OptiRamp PV Reduction 1) is the reduction due to excessive deviation in different regions; the optimal adjustment bottom reduction (OptiRamp Bottom Reduction 1) is the reduction due to too low power (floating); the optimal adjustment net set point gain (OptiRamp Net RampSP Gain 1) is the weighted sum of the three correction actions (for example, the net gain is a multiplier between 0.0 and 1.0 on the desired adjustment SP to reduce the adjustment rate, where 1.0 is no reduction and 0.5 is a 50% reduction); and the optimal adjustment set point reduction (OptiRamp SP Reduction 1) is the weighted sum of the three correction actions (for example, the net gain is a multiplier between 0.0 and 1.0 on the desired adjustment SP to reduce the adjustment rate, where 1.0 is no reduction and 0.5 is a 50% reduction); 1)), initially there may be a spike because the region may deviate from the SP modulation.

As used herein, the term deviation threshold generally captures a variety of possible thresholds that define the difference between a measured value (e.g., zone temperature, heater temperature) and another value (e.g., a temperature set point, a temperature of another zone, etc.). In one form, the deviation threshold utilized to monitor zone-to-zone drift/deviation in variable ramp-up control and variable ramp-down control may be the same or different thresholds. In one form, for variable ramp-down control, the deviation thresholds for zone-to-zone drift and set point deviation may be the same or different. Additionally, the deviation threshold may be set as a single absolute value (e.g., 5°C) or as a range (e.g., ±5°C). The actual value of the deviation threshold is application specific and is therefore not limited to any specific value provided herein.

Unless otherwise expressly stated herein, all numerical values indicating mechanical/thermal properties, composition percentages, dimensions and/or tolerances or other characteristics within the scope of the present disclosure are to be understood as modified by the word "about" or "approximately". Such modifications are desirable for a variety of reasons, including: industrial practice; material, manufacturing, and assembly tolerances; and testing capabilities.

The phrase at least one of A, B, and C, when used herein, should be interpreted using a non-exclusive logical "or" to mean a logical (A or B or C), and should not be interpreted to mean "at least one A, at least one B, and at least one C".

In this application, the term "controller" may be replaced by the term "circuit". The term "controller" may refer to, be a part of, or may include: an application specific integrated circuit (ASIC); a digital, analog, or hybrid analog/digital discrete circuit; a digital, analog, or hybrid analog/digital integrated circuit; a combination logic circuit; a field programmable gate array (FPGA); a processor circuit (shared, dedicated, or grouped) that executes program code; a memory circuit (shared, dedicated, or grouped) that stores program code executed by the processor circuit; other suitable hardware components that provide the functionality described; or a combination of some or all of the above, such as in a single-chip system.

The term code may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, data structures, and/or objects. The term memory circuit is a subset of the term computer-readable medium. As used herein, the term computer-readable medium does not encompass transient electrical or electromagnetic signals propagated through a medium (such as on a carrier wave); thus, the term computer-readable medium may be considered tangible and non-transitory.

The descriptions of this disclosure are merely exemplary in nature, and therefore, variations that do not depart from the substance of this disclosure are intended to fall within the scope of this disclosure. Such variations should not be considered as departing from the spirit and scope of this disclosure.

200: VRRT control routine, routine

202,204,206,208,210,212: Steps

Claims (23)

A method of controlling the temperature of a heater, the heater including a resistive heating element, the method comprising: applying electrical power to the resistive heating element of the heater at a variable modulation rate to increase the temperature of the heater to a desired temperature set point, wherein the variable modulation rate is set to a desired modulation rate; monitoring a current flowing through the resistive heating element of the heater; and in response to the current being greater than a lower limit of a current limit band, reducing the variable modulation rate from the desired modulation rate to an allowable modulation rate based on a variable reduction factor, wherein an upper limit of the current limit band is set to a system current limit. The method of claim 1, wherein reducing the variable modulation rate further comprises: determining a reduction amount of the desired modulation rate based on the variable reduction factor, wherein the variable reduction factor increases as the current of the resistive heating element approaches the system current limit; and reducing the variable modulation rate to the allowable modulation rate by the reduction amount. The method of claim 2, wherein the variable reduction factor provides a proportional reduction in the variable modulation rate based on the proximity of the current to the system current limit. The method of claim 2, wherein the reduction is determined based on the following: RedAmt = (DesiredRate * %Reduction * RedFactor) Equation 2.....%Reduction = 1.0-((ZoneCurLim-MeasuredCurrent)/CurrentBand) Where: "RedAmt" is the reduction, "DesiredRate" is the desired modulation rate, "RedFactor" is the amount by which the variable modulation rate is reduced when the current is equal to the system current limit, "ZoneCurLim" is the maximum current limit of the resistive heating element, wherein the measured current is equal to the system current limit, "MeasuredCurrent" is the measured current, and "CurrentBand" is the current limit band. The method of claim 1, wherein the heater comprises a plurality of resistive heating elements defining a plurality of regions, wherein each of the plurality of regions has a defined variable modulation rate. The method of claim 5, wherein: the current at each of the plurality of regions is monitored, and in response to at least one of the plurality of regions having a current greater than the lower limit of the current limit band, the variable modulation rate is reduced from the desired modulation rate to the allowable modulation rate. The method of claim 6 further comprises: determining a reduction amount for the at least one region having a current greater than the lower limit of the current limit band based on the variable reduction factor, wherein the variable reduction factor increases as the current approaches the system current limit; and reducing the variable modulation rate of each of the plurality of regions based on the reduction amount to obtain the allowable modulation rate of each of the plurality of regions. The method of claim 5 further comprises: monitoring a zone temperature of each of the plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; and in response to the difference being greater than the deviation threshold, adjusting the variable modulation rate of the first zone, the second zone, or a combination thereof, wherein a zone of the first zone and the second zone having a higher zone temperature is set as a hot zone, and another zone of the first zone and the second zone is a cold zone. As in the method of claim 8, in order to adjust the variable modulation rate, the method further comprises: reducing the variable modulation rate of the hot area; increasing the variable modulation rate of the cold area, or a combination thereof. The method of claim 8, wherein in order to adjust the variable modulation rate, the method further comprises: reducing the variable modulation rate of the hot zone to zero to maintain the zone temperature of the hot zone until the difference is no longer greater than the deviation threshold; and increasing the variable modulation rate of the hot zone in response to the difference being less than the deviation threshold. The method of claim 1 further comprises: setting the variable modulation rate to a sliding control rate, wherein the sliding control rate is less than the desired modulation rate; and increasing the variable modulation rate to the desired modulation rate in response to a sliding condition being satisfied, wherein the sliding condition comprises: after a predetermined time, the temperature of the heater is equal to a sliding temperature set point that is less than the desired temperature set point, or a combination thereof. The method of claim 1 further comprises: determining whether the temperature of the heater is at a temperature approaching a critical value, wherein the temperature approaching the critical value is less than the desired temperature set point; and in response to the temperature of the heater being at the temperature approaching the critical value, reducing the variable modulation rate to a near modulation rate, wherein the near modulation rate is less than the desired modulation rate. A control system for controlling power supplied to a heater, the heater including a resistive heating element, the control system comprising: a processor; and a non-transitory computer-readable medium including instructions executable by the processor, wherein the instructions include: determining the amount of power to be supplied to the resistive heating element of the heater based on a variable modulation rate to increase the temperature of the heater to a desired temperature set point, wherein the variable modulation rate is set to a desired modulation rate; monitoring a current flowing through the resistive heating element of the heater; and in response to the current being greater than a lower limit of a current limit band, reducing the variable modulation rate from the desired modulation rate to an allowable modulation rate based on a variable reduction factor, wherein an upper limit of the current limit band is set to a system current limit. The control system of claim 13, wherein the instructions further include: determining a reduction amount of the desired modulation rate based on the variable reduction factor, wherein the variable reduction factor increases as the current of the resistive heating element approaches the system current limit; and reducing the variable modulation rate by the reduction amount to obtain the allowable modulation rate. A control system as claimed in claim 14, wherein the variable reduction factor provides a proportional reduction in one of the variable modulation rates based on the proximity of the current to the system current limit. A control system as claimed in claim 13, wherein the heater comprises a plurality of resistive heating elements defining a plurality of regions, wherein each of the plurality of regions has a defined variable modulation rate. The control system of claim 16, wherein: the current at each of the plurality of regions is monitored, and in response to at least one of the plurality of regions having a current greater than the lower limit of the current limit band, the variable modulation rate is reduced from the desired modulation rate to the allowable modulation rate. The control system of claim 17, wherein the instructions further include: determining a reduction amount for the at least one region having a current greater than the lower limit of the current limit band based on the variable reduction factor, wherein the variable reduction factor increases as the current approaches the system current limit; and reducing the variable modulation rate of each of the plurality of regions based on the reduction amount to obtain the allowable modulation rate of each of the plurality of regions. The control system of claim 17, wherein the instructions further include: monitoring a zone temperature of each of the plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; and in response to the difference being greater than the deviation threshold, adjusting the variable modulation rate of the first zone, the second zone, or a combination thereof, wherein a zone of the first zone and the second zone having a higher zone temperature is set as a hot zone, and another zone of the first zone and the second zone is a cold zone. As in claim 19, the control system, wherein in order to adjust the variable modulation rate, the instructions further include: reducing the variable modulation rate of the hot zone; increasing the variable modulation rate of the cold zone; or a combination thereof. The control system of claim 19, wherein the instructions further include: reducing the variable modulation rate of the hot zone to zero to maintain the zone temperature of the hot zone until the difference is no longer greater than the deviation threshold; and increasing the variable modulation rate in response to the difference being less than the deviation threshold. The control system of claim 13, wherein the instructions further include: setting the variable modulation rate to a sliding control rate, wherein the sliding control rate is less than the desired modulation rate; and In response to a sliding condition being satisfied, increasing the variable modulation rate to the desired modulation rate, wherein the sliding condition includes: after a predetermined time, the temperature of the heater is equal to a sliding temperature set point less than the desired temperature set point, or a combination thereof. The control system of claim 13, wherein the instructions further include: determining whether the temperature of the heater is at a temperature approaching a critical value, wherein the temperature approaching the critical value is less than the desired temperature set point; and in response to the temperature of the heater being at the temperature approaching the critical value, reducing the variable modulation rate to a near modulation rate, wherein the near modulation rate is less than the desired modulation rate.